I did a search to discover what I'm missing -- and it turns out at that I'm not "missing" anything at all. This is a big, general problem, and the Apple boards are overwhelmed by people screaming bloody murder about this In fact, it turns out that for some people, old podcasts that they had archived and been saving are done gone. I skimmed through probably 75 posts with the same complaint before I gave up. There were pages and pages more unread.
Not a great job with the upgrade, I'd say...